1. Field of the Invention
This invention relates to a fluorescent printer head or a write head for an optical printer which comprises a vacuum fluorescent tube using the principle of a fluorescent display tube, and more particularly to a fluorescent printer head of the multi-electrode tube type.
2. Description of the Prior Art
U.S. Pat. No. 4,730,203 discloses a fluorescent printer head with a tetrode-type structure which includes two control electrodes between anodes and cathodes. FIG. 8 is a partly cutaway plan view showing such a conventional structure, generally indicated by reference numeral 100. The printer head 100 includes a substrate 101, on which a plurality of strip-like anode conductors 102 are arranged parallel to one another. On the portion of each of the anode conductors 102 opposite to a first control electrode, a continuous phosphor layer 103 is deposited, resulting in (an) anodes 104. The first control electrode will be described below.
A second control electrode 105 is located above the anodes 104 through an insulating layer (not shown) at the periphery of the substrate 101 and is thus spaced above the substrate 101 by a predetermined distance. The second control electrode 105 comprises a plurality of flat grid plates 105a extending obliquely across the anode conductors 102. The grid plates 105a are parallel to one another and are electrically separate from one another. Each of the grid plates 105a has a central slit-like aperture or slit also extending obliquely across the anode conductors 102, so that the portion of each of the phosphor layers defined by each of the slits 106 forms a luminous dot which emits light due to the impingement of electrons.
Insulating spacers (not shown) are located at both edges of the second control electrode 105, the first control electrode 108 is located above the second control electrode 105 with the insulating spacers between them. The first control electrode 108 is formed from a single metal plate which is larger than the second control electrode 105 and is formed with slits 109 which are larger than the slits 106 of the second control electrode 105 but which correspond positionally to the slits 106. In this way, the portions of the phosphor layers 103 of the anodes 104 exposed between adjacent grid plates 105a are hidden by the first control electrode 108, so that effective luminous sections are constituted by only the luminous dots 107.
The ends of the first control electrode 108 defined in the longitudinal direction of the anodes 104 have a configuration corresponding to the grid plates 105a at the ends of the second control electrode 105 and the slits 106 and 109; thus, each end is formed obliquely across the longitudinal direction of the anodes 104. The ends of the phosphor layers 103 of the anodes 104 are progressively displaced across the ends of the first control electrode 108, so that as a whole, the ends are arranged obliquely. Also, the phosphor layers 103 are formed so that they do not protrude laterally beyond the first control electrode 108, and so only the anode conductors 102 are exposed beyond the ends of the first control electrode 108. Alternatively, the phosphor layers 103 can be deposited over the entire respective anode conductors 102 and those portions of the phosphor layers protruding beyond the first control electrode 108 can be covered with a coating layer, in order to prevent any light being emitted from these portions.
The conventional printer head also includes filamentary cathodes 200 extending above the first control electrode 108. A casing comprising side plates 201 and a front cover 202 is bonded on to the substrate 101 by means of a sealing material, resulting in an envelope, which is then evacuated to form a high vacuum atmosphere therein.
Electrodes, such as grid terminals and the like, are led out from the so-formed vacuum envelope and are then connected to a driver circuit (not shown). The anodes 104 are scanned with a time-division pulse signal and a positive display pulse signal is supplied to the desired grid plate(s) of the second control electrode 105 in synchronism with the scanning, resulting in the luminous dots of the phosphor layers 103 selectively omitting light as desired.
This conventional fluorescent printer head 100 is located adjacent a photosensitive drum in a printer so that the anodes 104 are parallel to the photosensitive drum. When the timing of luminescence of the luminous dots is suitably adjusted depending on the rotation of the photosensitive drum, lights emitted from the luminous dots 107 are continuously irradiated onto the surface of the photosensitive drum so as to form a straight line on the drum, parallel to its axis resulting in characters, figures or the like being formed on the drum surface.
The conventional fluorescnet printer head 100 constructed as described above causes a difference in luminance to occur between a luminous dot 107 at the central region of each anode (in its longitudinal direction) and a luminous dot 107 at either end of the anode 104 as shown in FIG. 7. In FIG. 7, the x-axis indicates the number on grid plates 105a arranged in the longitudinal direction of the anodes 104 and the y-axis indicates the relative luminance of each of the luminous dots 107. As can be seen from FIG. 7, in the conventional printer head 100, the luminance of the luminous dots at the ends of each anode 104 is decreased compared with that of a luminous dot in the central region of the anode 14, resulting in a nonuniformity of luminance. This nonuniformity in the luminance renders the density of an image nonuniform, so that the quality of printing is unsatisfactory.
Also, in the conventional printer head 100, the ends of each of the phosphor layers 103 are formed obliquely to conform with the ends of the first and second control electrodes 105 and 108. Alternatively, phosphor layer 103 must be formed over the entire corresponding anode conductor and the unnecessary portion of the phosphor layer 103 must be covered with a coating layer. However, each of the approaches causes manufacturing of the printer head to be troublesome and complicated, leading to an increase in manufacturing costs.